Configuring Redistribution | Routing TCP/IP, Volume 1 (2nd Edition)

Redistribution is configured in two steps:

Step 1.	In the routing protocol configuration that is to receive the redistributed routes, use the redistribute command to specify the source of the routes.
Step 2.	Specify the metric to be assigned to the redistributed routes.

For example, the EIGRP configuration of Lajoie in Figure 11-7 is displayed in Example 11-12.

Figure 11-7. Configuring redistribution sample network.

Example 11-12. Lajoie's EIGRP configuration shows OSPF being redistributed into the EIGRP process.

router eigrp 1  redistribute ospf 1 metric 10000 100 255 1 15003  passive-interface Ethernet1  network 172.20.0.0

This configuration redistributes routes discovered by OSPF process 1 into EIGRP process 1. The metric portion of the command assigns EIGRP metrics to the routes. In order, the numbers specify the following:

Bandwidth, in kilobits per second
Delay, in tens of microseconds
Reliability, as a fraction of 255
Load, as a fraction of 255
MTU, in octets

The OSPF configuration of Lajoie is in Example 11-13

Example 11-13. Lajoie's OSPF configuration shows the redistribution of EIGRP routes into OSPF.

router ospf 1  redistribute eigrp 1 metric 30 metric-type 1 subnets  network 172.20.11.2 0.0.0.0 area 0

This configuration redistributes routes discovered by EIGRP process 1 into OSPF process 1. The metric portion of the command assigns an OSPF cost of 30 to each redistributed route. The redistribution makes Lajoie an ASBR in the OSPF domain, and the redistributed routes are advertised as external routes. The metric-type portion of the command specifies that the external type of the routes is E1. The subnets keyword, used only when redistributing routes into OSPF, specifies that subnet details will be redistributed. Without it, only major network addresses are redistributed. More will be said about the subnets keyword in the case studies.

An alternative method of specifying the metrics to be assigned to redistributed routes is to use the default-metric command. For example, the previous OSPF configuration can also be written as in Example 11-14.

Example 11-14. Lajoie's OSPF configuration uses the default-metric command to assign an OSPF metric to all routes redistributed into OSPF.

router ospf 1  redistribute eigrp 1 metric-type 1 subnets  default-metric 30  network 172.20.11.2 0.0.0.0 area 0

The results of this configuration are exactly the same as the previous configuration. The default-metric command is useful when routes are being redistributed from more than one source. For example, suppose router Lajoie in Figure 11-7 was running not only EIGRP and OSPF but also RIP. The OSPF configuration might be as displayed in Example 11-15.

Example 11-15. Lajoie's OSPF configuration uses the default-metric command to assign an OSPF metric to routes redistributed from both EIGRP and RIP.

router ospf 1  redistribute eigrp 1 metric-type 1 subnets  redistribute rip metric-type 1 subnets default-metric 30  network 172.20.11.2 0.0.0.0 area 0

Here an OSPF cost of 30 will be assigned to all EIGRP- and RIP-learned routes.

The two methods of assigning metrics can also be used with each other. For example, suppose Lajoie were to be configured to redistribute OSPF and RIP into EIGRP, but that RIP routes were to be advertised with a different set of metrics than the OSPF routes. The configuration might be like Example 11-16.

Example 11-16. Lajoie's EIGRP configuration uses a metric with the redistribution command to assign metrics to addresses redistributed from RIP, and uses the default metric command to assign the EIGRP metric to all other redistributed addresses.

router eigrp 1  redistribute ospf 1  redistribute rip metric 50000 500 255 1 1500  default-metric 10000 100 255 1 1500  passive-interface Ethernet1  network 172.20.0.0

The metrics assigned using the metric keyword with the redistribute command take precedence over metrics assigned with the default-metric command. RIP-learned routes will be advertised into EIGRP with the metrics specified on the redistribute rip line, and the OSPF-learned routes will be advertised with the metrics specified by the default-metric command.

If neither the metric keyword nor the default-metric command specifies a metric, the metric will default to 20 for routes redistributed into OSPF and to 0 for routes redistributed into other protocols. The 0 metric will be understood by IS-IS, but not by RIP, whose hop count must be between 1 and 16. The 0 metric is also incompatible with the EIGRP multi-metric format. These two protocols must have the appropriate metrics assigned to any redistributed routes, or redistribution will not work. The following case studies examine techniques for configuring redistribution into the various IP IGPs. In addition, they are arranged so that the more generic issues of redistributing classful to classful, classless to classless, and classless to classful can be examined.

Case Study: Redistributing IGRP and RIP

In the network of Figure 11-8, Ford is running IGRP, and Berra is running RIP. Mantle's routing configuration is shown in Example 11-17.

Figure 11-8. Ford is running IGRP, and Berra is running RIP. Mantle is performing redistribution.

Example 11-17. Mantle is running both IGRP and RIP and performing redistribution between the two protocols, using both the default metric and the metric assigned with the redistribution command.

router rip  redistribute igrp 1 metric 5  passive-interface Ethernet1  network 10.0.0.0 ! router igrp 1  redistribute rip  default-metric 1000 100 255 1 1500  passive-interface Ethernet0  network 10.0.0.0

Both methods of assigning metrics are used here for demonstration purposes. In most cases, a redistribution scheme as simple as this will use one method or the other.

Notice that Mantle is also connected to a stub network (192.168.10.0/24). In this case, the stub network should be advertised into the IGRP domain, but not into the RIP domain. One way to accomplish this configuration is to simply add the appropriate network statement under IGRP. However, doing so will create unnecessary IGRP broadcasts on the stub network. Another way to achieve the desired configuration is to use redistribution as in Example 11-18.

Example 11-18. Mantle redistributes the connected stub network into IGRP.

router rip  redistribute igrp 1 metric 5  passive-interface Ethernet1  network 10.0.0.0 ! router igrp 1  redistribute connected  redistribute rip  default-metric 1000 100 255 1 1500  passive-interface Ethernet0  network 10.0.0.0

The redistribute connected command will redistribute all directly connected networks. If network 192.168.10.0/24 is to be advertised into the IGRP domain and the RIP domain, the configuration would be as in Example 11-19.

Example 11-19. Mantle's configuration shows the connected stub network advertised into both RIP, assigning the metric during redistribution, and IGRP, assigning the metric using the default metric command.

router rip  redistribute connected metric 5  redistribute igrp 1 metric 5  passive-interface Ethernet1  network 10.0.0.0 ! router igrp 1  redistribute connected  redistribute rip  default-metric 1000 100 255 1 1500  passive-interface Ethernet0  network 10.0.0.0

Case Study: Redistributing EIGRP and OSPF

The network of Figure 11-9 has an OSPF domain and two EIGRP domains. Router Hodges is running OSPF process 1. Podres is running EIGRP process 1, and on Snider and Campanella, EIGRP process 2 is running.

Figure 11-9. Hodges is running OSPF, and Podres is running EIGRP 1. Snider and Campanella are running EIGRP 2.

Robinson has the configuration in Example 11-20.

Example 11-20. Robinson is redistributing the EIGRP1, EIGRP2, and OSPF processes.

router eigrp 1  redistribute ospf 1 metric 1000 100 1 255 1500  redistribute eigrp 2 passive-interface FastEthernet0/0  network 192.168.3.0 ! router eigrp 2  redistribute ospf 1 metric 1000 100 1 255 1500  redistribute eigrp 1 network 192.168.4.0  network 172.16.0.0 ! router ospf 1  redistribute eigrp 1  redistribute eigrp 2 metric 100  default-metric 50  network 192.168.3.33 0.0.0.0 area 0

Notice that although redistribution must be configured between the EIGRP processes, no metrics are configured. The processes use the same metrics, so the metrics can be tracked accurately across the redistribution boundary. Example 11-21 shows Podres' route table. The redistributed routes are tagged as EIGRP external routes.

Example 11-21. The route table of Podres in Figure 11-9.

Podres#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E  EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o  ODR        P - periodic downloaded static route Gateway of last resort is not set 172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks D EX    172.16.2.20/30 [170/2195456] via 192.168.3.129, 00:00:26, Ethernet0 D EX    172.16.0.0/16 [170/2195456] via 192.168.3.129, 00:00:26, Ethernet0 D EX    172.16.1.0/24 [170/2221056] via 192.168.3.129, 00:00:26, Ethernet0      192.168.4.0/24 is variably subnetted, 3 subnets, 3 masks D EX    192.168.4.72/29 [170/2221056] via 192.168.3.129, 00:00:26, Ethernet0 D EX    192.168.4.4/30 [170/2195456] via 192.168.3.129, 00:00:26, Ethernet0 D EX    192.168.4.0/24 [170/2195456] via 192.168.3.129, 00:00:27, Ethernet0 D EX 192.168.1.0/24 [170/2611200] via 192.168.3.129, 00:00:28, Ethernet0 C    192.168.2.0/24 is directly connected, Ethernet3      192.168.3.0/24 is variably subnetted, 7 subnets, 2 masks D EX    192.168.3.96/27 [170/2611200] via 192.168.3.129, 00:00:28, Ethernet0 D EX    192.168.3.64/27 [170/2611200] via 192.168.3.129, 00:00:28, Ethernet0 D       192.168.3.32/27 [90/284160] via 192.168.3.129, 00:00:35, Ethernet0 D       192.168.3.0/24 is a summary, 00:07:18, Null0 C       192.168.3.192/27 is directly connected, Ethernet2 C       192.168.3.160/27 is directly connected, Ethernet1 C       192.168.3.128/27 is directly connected, Ethernet0 Podres#

Example 11-22 shows Hodges' route table, which has some problems. Recall from Chapter 8, "OSPFv2," that routes redistributed into OSPF are either type 1 (E1) or type 2 (E2) external routes. The only routes that seem to have been redistributed, indicated by the E2 tag, are the major network addresses 192.168.2.0/24, 172.16.0.0/16, and 192.168.4.0/24. The reason for this is the absence of the subnets keyword in Robinson's redistribution statements. Without this keyword, only major network addresses within the non-OSPF process will be redistributed.

Example 11-22. The route table of Hodges contains only major network redistributed routes, indicated by the E2 tag.

Hodges#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2        ia - IS-IS inter area, * - candidate default, U - per-user static route        o - ODR, P - periodic downloaded static route Gateway of last resort is not set O E2 172.16.0.0/16 [110/100] via 192.168.3.33, 00:24:41, FastEthernet0/0 O E2 192.168.4.0/24 [110/100] via 192.168.3.33, 00:24:41, FastEthernet0/0 C    192.168.1.0/24 is directly connected, Serial0/0 O E2 192.168.2.0/24 [110/50] via 192.168.3.33, 00:02:57, FastEthernet0/0      192.168.3.0/27 is subnetted, 3 subnets C       192.168.3.96 is directly connected, Serial0/0 C       192.168.3.64 is directly connected, Serial0/0 C       192.168.3.32 is directly connected, FastEthernet0/0 Hodges#

Robinson's configuration is changed to include the subnets keyword as displayed in Example 11-23.

Example 11-23. Robinson uses the subnets keyword when redistributing routes into OSPF so that subnets in addition to major network addresses will be redistributed.

router eigrp 1  redistribute ospf 1 metric 1000 100 1 255 1500  redistribute eigrp 2  passive-interface FastEthernet0/0  network 192.168.3.0 ! router eigrp 2  redistribute ospf 1 metric 1000 100 1 255 1500  redistribute eigrp 1  network 192.168.4.0  network 172.16.0.0 ! router ospf 1  redistribute eigrp 1 subnets  redistribute eigrp 2 metric 100 subnets  default-metric 50 network 192.168.3.33 0.0.0.0 area 0

As a result of this change, the subnets in Figure 11-9 are in Hodges' route table (Example 11-24).

Example 11-24. After the subnets keyword is added to Robinson's redistribution configuration, Hodges has knowledge of all subnets.

Hodges#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2        ia - IS-IS inter area, * - candidate default, U - per-user static route        o - ODR, P - periodic downloaded static route Gateway of last resort is not set      172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks O E2    172.16.2.20/30 [110/100] via 192.168.3.33, 00:00:59, FastEthernet0/0 O E2    172.16.0.0/16 [110/100] via 192.168.3.33, 00:30:20, FastEthernet0/0 O E2    172.16.1.0/24 [110/100] via 192.168.3.33, 00:00:59, FastEthernet0/0      192.168.4.0/24 is variably subnetted, 3 subnets, 3 masks O E2    192.168.4.72/29 [110/100] via 192.168.3.33, 00:00:59, FastEthernet0/0 O E2    192.168.4.4/30 [110/100] via 192.168.3.33, 00:01:00, FastEthernet0/0 O E2    192.168.4.0/24 [110/100] via 192.168.3.33, 00:30:22, FastEthernet0/0 C    192.168.1.0/24 is directly connected, Serial0/0 O E2 192.168.2.0/24 [110/50] via 192.168.3.33, 00:08:38, FastEthernet0/0      192.168.3.0/27 is subnetted, 6 subnets C       192.168.3.96 is directly connected, Serial0/0 C       192.168.3.64 is directly connected, Serial0/0 C       192.168.3.32 is directly connected, FastEthernet0/0 O E2    192.168.3.192 [110/50] via 192.168.3.33, 00:01:12, FastEthernet0/0 O E2    192.168.3.160 [110/50] via 192.168.3.33, 00:01:12, FastEthernet0/0 O E2    192.168.3.128 [110/50] via 192.168.3.33, 00:01:12, FastEthernet0/0 Hodges#

By default, external routes are redistributed into OSPF as type 2 routes. As discussed in Chapter 8, E2 routes include only the external cost of the route. This fact can be important when a single destination is reachable by more than one external route, as shown in Figure 11-10. In this network, one router is redistributing the route to 10.2.3.0/24 with a cost of 50, and the other router is redistributing a different route to the same destination with a cost of 100. If the routes are advertised as E2, the costs of the links within the OSPF domain will not be added. As a result, the router internal to the OSPF domain will choose route 1 to reach 10.2.3.0/24.

Figure 11-10. If the routes to 10.2.3.0/24 are advertised as E2, route 1 will have a cost of 50 and route 2 will have a cost of 100. If the routes are advertised as E1, route 1 will have a cost of 150 and route 2 will have a cost of 110.

If the routes to 10.2.3.0/24 in Figure 11-10 are redistributed as E1, the costs of the links within the OSPF domain will be added to the redistributed costs. As a result, the router internal to the OSPF domain would choose route 2, with a cost of 110 (100 + 10) over route 1, with a cost of 150 (50 + 100).

Robinson, in Figure 11-9, is redistributing EIGRP 1 with a cost of 50 and redistributing EIGRP 2 with a cost of 100. Example 11-24 shows that, at Hodges, the routes to the EIGRP 1 subnets still have a cost of 50, and the routes to the EIGRP 2 subnets still have a cost of 100. The cost of the FastEthernet link between Hodges and Robinson has not been added to the routes.

To redistribute routes into OSPF as E1, the keyword metric-type 1 is added to the redistribution command. In the configuration in Example 11-25, Robinson continues to redistribute EIGRP 1 as E2, but EIGRP 2 is redistributed as E1.

Example 11-25. Robinson configures routes redistributed into OSPF from EIGRP2 as external type 1 routes.

router eigrp 1  redistribute ospf 1 metric 1000 100 1 255 1500  redistribute eigrp 2  passive-interface FastEthernet0/0  network 192.168.3.0 ! router eigrp 2  redistribute ospf 1 metric 1000 100 1 255 1500  redistribute eigrp 1  network 192.168.4.0  network 172.16.0.0 ! router ospf 1  redistribute eigrp 1 subnets  redistribute eigrp 2 metric 100 metric-type 1 subnets  default-metric 50 network 192.168.3.33 0.0.0.0 area 0

Example 11-26 shows Hodges's route table after Robinson is reconfigured. All the routes to destinations within the EIGRP 1 domain still have a cost of 50, but the routes to destinations within the EIGRP 2 domain now have a cost of 101 (the redistributed cost plus the default cost of 1 for the FastEthernet link between Robinson and Hodges).

Example 11-26. Robinson's configuration has been changed so that the subnets of 192.168.4.0 and 172.16.0.0 are being advertised as type 1 external routes.

Hodges#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2        ia - IS-IS inter area, * - candidate default, U - per-user static route        o - ODR, P - periodic downloaded static route Gateway of last resort is not set      172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks O E1    172.16.2.20/30 [110/101] via 192.168.3.33, 09:01:00, FastEthernet0/0 O E1    172.16.0.0/16 [110/101] via 192.168.3.33, 00:01:02, FastEthernet0/0 O E1    172.16.1.0/24 [110/101] via 192.168.3.33, 09:01:00, FastEthernet0/0      192.168.4.0/24 is variably subnetted, 3 subnets, 3 masks O E1    192.168.4.72/29 [110/101] via 192.168.3.33, 00:00:57, FastEthernet0/0 O E1    192.168.4.4/30 [110/101] via 192.168.3.33, 00:01:03, FastEthernet0/0 O E1    192.168.4.0/24 [110/101] via 192.168.3.33, 00:01:03, FastEthernet0/0 C    192.168.1.0/24 is directly connected, Serial0/0 O E2 192.168.2.0/24 [110/50] via 192.168.3.33, 09:01:01, FastEthernet0/0      192.168.3.0/27 is subnetted, 6 subnets C       192.168.3.96 is directly connected, Serial0/0 C       192.168.3.64 is directly connected, Serial0/0 C       192.168.3.32 is directly connected, FastEthernet0/0 O E2    192.168.3.192 [110/50] via 192.168.3.33, 09:01:06, FastEthernet0/0 O E2    192.168.3.160 [110/50] via 192.168.3.33, 09:01:06, FastEthernet0/0 O E2    192.168.3.128 [110/50] via 192.168.3.33, 09:01:06, FastEthernet0/0 Hodges#

Case Study: Redistribution and Route Summarization

The Cisco EIGRP, OSPFv2, OSPFv3, and IS-IS implementations have the capability to summarize redistributed routes. This case study examines summarization for EIGRP and OSPF for IPv4; the following two case studies examine OSPFv3 for IPv6 and IS-IS summarization.

The first thing to note is that summarization is useful only if the IP subnet addresses have been planned for summarization. For example, the subnets of 192.168.3.0 within the OSPF domain in Figure 11-9 all fall under the summary address 192.168.3.0/25. The subnets of the same major address within the EIGRP 1 domain all fall under the summary address 192.168.3.128/25. If subnet 192.168.3.0/27 were to be connected to Podres, that single destination would have to be advertised separately from the summary address. Although advertising such a single destination will have little adverse impact, advertising a large number of subnets outside of the range of the summary address will reduce the benefits of summarization.

The command summary-address specifies a summary address and mask to an OSPF process. Any more-specific subnet addresses that fall within the range of the specified summary address will be suppressed. Note that this command is used only to summarize external routes at ASBRs; summarization of internal OSPF routes at ABRs is accomplished with the area range command, as discussed in Chapter 8.

At Robinson in Figure 11-9, the EIGRP 1 subnets are summarized into the OSPF domain with 192.168.3.128/25, and the EIGRP 2 subnets are summarized with 172.16.0.0/16, as displayed in Example 11-27.

Example 11-27. Robinson summarizes external addresses advertised into OSPF.

router eigrp 1  redistribute ospf 1 metric 1000 100 1 255 1500  redistribute eigrp 2  passive-interface FastEthernet0/0  network 192.168.3.0 ! router eigrp 2  redistribute eigrp 1  network 192.168.4.0  network 172.16.0.0 ! router ospf 1  summary-address 192.168.3.128 255.255.255.128  summary-address 172.16.0.0 255.255.0.0  redistribute eigrp 1 subnets  redistribute eigrp 2 metric 100 metric-type 1 subnets  default-metric 50  network 192.168.3.33 0.0.0.0 area 0

Compare Example 11-28 with Example 11-26. In Example 11-28, Hodges' route table contains the specified summary addresses. The subnet addresses within the summary range have been suppressed at the redistribution point. Notice, also, that no summarization was configured for 192.168.4.0/24, so the subnets of that major address are still in the route table.

Example 11-28. Robinson is summarizing 192.168.3.128/25 and 172.16.0.0/16, so no more-specific subnets within those ranges appear in the route table of Hodges.

Hodges#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2        ia - IS-IS inter area, * - candidate default, U - per-user static route        o - ODR, P - periodic downloaded static route Gateway of last resort is not set O E1 172.16.0.0/16 [110/101] via 192.168.3.33, 00:11:55, FastEthernet0/0      192.168.4.0/24 is variably subnetted, 3 subnets, 3 masks O E1    192.168.4.72/29 [110/101] via 192.168.3.33, 00:03:56, FastEthernet0/0 O E1    192.168.4.4/30 [110/101] via 192.168.3.33, 00:11:55, FastEthernet0/0 O E1    192.168.4.0/24 [110/101] via 192.168.3.33, 00:11:55, FastEthernet0/0 C    192.168.1.0/24 is directly connected, Serial0/0 O E2 192.168.2.0/24 [110/50] via 192.168.3.33, 00:03:57, FastEthernet0/0      192.168.3.0/24 is variably subnetted, 4 subnets, 2 masks C       192.168.3.96/27 is directly connected, Serial0/0 C       192.168.3.64/27 is directly connected, Serial0/0 C       192.168.3.32/27 is directly connected, FastEthernet0/0 O E2    192.168.3.128/25 [110/50] via 192.168.3.33, 00:00:37, FastEthernet0/0

Summarization for EIGRP is interface-specific. That is, instead of specifying the summary address and mask under the routing process, they are specified under individual interfaces. This system provides the flexibility to advertise different summary routes out different interfaces of the same process. The command ip summary-address eigrp process-id specifies the summary address and mask and the EIGRP process into which the summary is to be advertised.

In the configuration in Example 11-29, Robinson will advertise summary addresses 192.168.3.0/25, 172.16.0.0/16, and 192.168.4.0/24 into EIGRP 1.

Example 11-29. Robinson summarizes routes advertised into EIGRP 1.

interface FastEthernet0/0  ip address 192.168.3.33 255.255.255.224 ! interface FastEthernet0/1  ip address 192.168.3.129 255.255.255.224  ip summary-address eigrp 1 192.168.3.0 255.255.255.128  ip summary-address eigrp 1 172.16.0.0 255.255.0.0  ip summary-address eigrp 1 192.168.4.0 255.255.255.0 ! interface Serial0/0.1  ip address 192.168.4.5 255.255.255.252  ip summary-address eigrp 2 192.168.3.0 255.255.255.0 ! interface Serial0/0.2  ip address 172.16.2.21 255.255.255.252  ip summary-address eigrp 2 192.168.0.0 255.255.0.0 ! router eigrp 1  redistribute ospf 1 metric 1000 100 1 255 1500  redistribute eigrp 2  passive-interface FastEthernet0/0  network 192.168.3.0 ! router eigrp 2  redistribute eigrp 1  network 192.168.4.0  network 172.16.0.0 ! router ospf 1  summary-address 192.168.3.128 255.255.255.128  summary-address 172.16.0.0 255.255.0.0  redistribute eigrp 1 subnets  redistribute eigrp 2 metric 100 metric-type 1 subnets  default-metric 50  network 192.168.3.33 0.0.0.0 area 0

Example 11-30 shows the route table of Podres. As with OSPF summarization, EIGRP summarization suppresses the advertisement of subnets within the summary range. Unlike OSPF, Podres' route table shows that summary routes advertised into EIGRP are not tagged as external routes.

Example 11-30. The route table of Podres, showing summary routes 192.168.3.0/25, 192.168.4.0/24, and 172.16.0.0/16.

Podres#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E  EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o  ODR        P - periodic downloaded static route Gateway of last resort is not set D    172.16.0.0/16 [90/2195456] via 192.168.3.129, 00:00:12, Ethernet0 D    192.168.4.0/24 [90/2195456] via 192.168.3.129, 00:00:12, Ethernet0 D EX 192.168.1.0/24 [170/2611200] via 192.168.3.129, 00:00:12, Ethernet0 C    192.168.2.0/24 is directly connected, Ethernet3      192.168.3.0/24 is variably subnetted, 6 subnets, 3 masks D       192.168.3.0/25 [90/284160] via 192.168.3.129, 00:00:12, Ethernet0 D       192.168.3.0/24 is a summary, 00:00:12, Null0 C       192.168.3.192/27 is directly connected, Ethernet2 C       192.168.3.160/27 is directly connected, Ethernet1 D EX    192.168.3.128/25 [170/2611200] via 192.168.3.129, 00:00:13, Ethernet0 C       192.168.3.128/27 is directly connected, Ethernet0 D EX 192.168.0.0/16 [170/284160] via 192.168.3.129, 00:00:13, Ethernet0 Podres#

Robinson is advertising EIGRP summary routes of 192.168.3.0/24 to Campanella and 192.168.0.0/16 to Snider. Example 11-31 shows Campanella's route table, and Example 11-32 shows Snider's route table.

Example 11-31. Campanella's route table after summarization is configured at Robinson.

Campanella#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E  EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o  ODR        P - periodic downloaded static route Gateway of last resort is not set D EX 172.16.0.0/16 [170/2681856] via 192.168.4.5, 00:35:12, Serial0.1      192.168.4.0/24 is variably subnetted, 3 subnets, 3 masks C       192.168.4.72/29 is directly connected, Ethernet0 C       192.168.4.4/30 is directly connected, Serial0.1 D EX    192.168.4.0/24 [170/2681856] via 192.168.4.5, 00:35:12, Serial0.1 D EX 192.168.1.0/24 [170/3097600] via 192.168.4.5, 00:35:12, Serial0.1 D EX 192.168.2.0/24 [170/2300416] via 192.168.4.5, 00:35:12, Serial0.1 D    192.168.3.0/24 [90/2172416] via 192.168.4.5, 00:35:13, Serial0.1 Campanella#

Example 11-32. Snider's route table after summarization is configured at Robinson.

Snider#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2        ia - IS-IS inter area, * - candidate default, U - per-user static route        o - ODR, P - periodic downloaded static route Gateway of last resort is not set      172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks C       172.16.2.20/30 is directly connected, Serial0/0.1 D EX    172.16.0.0/16 [170/2681856] via 172.16.2.21, 00:07:59, Serial0/0.1 C       172.16.1.0/24 is directly connected, Ethernet0/0 D    192.168.0.0/16 [90/2195456] via 172.16.2.21, 00:07:59, Serial0/0.1 Snider#

Looking again at Example 11-30, notice the entry for the summary route 192.168.3.128/25. This entry might surprise you because that summary address is advertised into OSPF, not EIGRP. Notice also that the route is marked as an external route, indicating that it was redistributed into the EIGRP domain. What has happened is that the summary route was advertised into OSPF and was then redistributed back into EIGRP from the OSPF domain. Hence, the unexpected entry at Podres. This is the same reason why you see 172.16.0.0/16 and 192.168.4.0/24 as external routes on Campanella. These summary addresses are advertised to Podres, in EIGRP 1, and then redistributed back into EIGRP 2. Snider's route table shows the route to 172.16.0.0/16, but not 192.168.4.0/24, because Snider has an entry for the summarized address 192.168.0.0/16.

Now suppose subnet 192.168.3.192/27 were to become inaccessible. Podres would forward packets destined for that subnet to the less-specific route 192.168.3.128/25. The packet would be sent into the OSPF domain, where you might expect the summary route 192.168.3.128/25 to cause the packet to be sent right back to Podres.

In fact, this situation will not occur.

Robinson's route table (Example 11-33) has numerous entries for summary routes that show interface Null0 as a connected interface. The null interface is a software-only interface to nowherepackets routed to it are dropped. With some exceptions,^[2] whenever a router generates a summary address, the router will also create a route for that address that goes to the null interface. If Robinson receives a packet destined for 192.168.3.192/27 and that subnet is no longer reachable, the router will forward the packet to the null interface. The routing loop is broken in one hop.

^[2] In some older versions of IOS, OSPF inter-area summarization, for example, does not automatically create a summary route to the null interface. It must be configured statically.

Example 11-33. Robinson's route table. Because the router is originating many summary routes, there are many entries for summaries whose connected interface is Null0. This is a safeguard against routing loops.

Robinson#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E  EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o  ODR        P - periodic downloaded static route Gateway of last resort is not set      172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks C       172.16.2.20/30 is directly connected, Serial0/0.2 D       172.16.0.0/16 is a summary, 00:19:44, Null0 D       172.16.1.0/24 [90/2195456] via 172.16.2.22, 00:17:08, Serial0/0.2      192.168.4.0/24 is variably subnetted, 3 subnets, 3 masks D       192.168.4.72/29 [90/2172416] via 192.168.4.6, 00:15:17, Serial0/0.1 C       192.168.4.4/30 is directly connected, Serial0/0.1 D       192.168.4.0/24 is a summary, 00:20:53, Null0 O    192.168.1.0/24 [110/74] via 192.168.3.34, 00:21:14, FastEthernet0/0 D    192.168.2.0/24 [90/665600] via 192.168.3.130, 00:19:41, FastEthernet0/1      192.168.3.0/24 is variably subnetted, 8 subnets, 3 masks O       192.168.3.96/27 [110/74] via 192.168.3.34, 00:21:15, FastEthernet0/0 O       192.168.3.64/27 [110/74] via 192.168.3.34, 00:21:15, FastEthernet0/0 C       192.168.3.32/27 is directly connected, FastEthernet0/0 D       192.168.3.0/24 is a summary, 00:19:01, Null0 D       192.168.3.0/25 is a summary, 00:19:47, Null0 D       192.168.3.192/27 [90/665600] via 192.168.3.130, 00:19:42, FastEthernet0/1 D       192.168.3.160/27 [90/665600] via 192.168.3.130, 00:19:42, FastEthernet0/1 C       192.168.3.128/27 is directly connected, FastEthernet0/1 D    192.168.0.0/16 is a summary, 00:19:00, Null0 Robinson#

Summary routes to null interfaces are very helpful for preventing loops, and their use is described in greater detail in Chapter 12, "Default Routes and On-Demand Routing." However, the redistribution of incorrect routing information should not be allowed to happen at all. Suppose that instead of being one hop away from Robinson, Podres is 10 hops away. The misdirected packet would have to travel a long way before being dropped. This example demonstrates the need to carefully regulate route advertisements when using mutual redistributionthat is, when two routing protocols are redistributing their routes into each other. In such cases the use of route filters, described in Chapter 13, "Route Filtering," or route maps, as described in Chapter 14, "Route Maps," is essential.

The previous scenario also demonstrates the trade-off of using summarization. Although the size of the route table is reduced, saving memory and processor cycles, route precision is also reduced. As the network grows more complex, that loss of detail increases the possibility of routing errors.

Case Study: Redistributing OSPFv3 and RIPng

IPv6 is running on the network in Figure 11-11. Router Griffey is the connection point between three distinct networks. Two networks are running OSPFv3, the other is running RIPng. Routes are redistributed between the routing processes by Griffey.

Figure 11-11. IPv6 is being routed with both OSPFv3 and RIPng, and a single router is redistributing between the routing protocols.

Griffey's configuration is displayed in Example 11-34.

Example 11-34. Griffey is configured with IPv6, OSPFv3 process 1, OSPFv3 process 2, and RIPng, and is redistributing between the processes.

interface Ethernet 0/0  ipv6 address 2001:db8:0:21::1/64  ipv6 rip Mariners enable ! interface Serial 0/0.1 point-to-point  ipv6 address 2001:db8:0:31::1/64  ipv6 ospf 1 area 1 ! interface Serial 0/0.4 point-to-point  ipv6 address 2001:db8:0:30::1/64  ipv6 ospf 2 area 20 ! ipv6 router ospf 1  redistribute rip Mariners metric-type 1 tag 4 ! ipv6 router ospf 2  redistribute rip Mariners ! ipv6 router rip Mariners  redistribute ospf 1 metric 5  redistribute ospf 2 metric 10

Griffey is redistributing routes known via the RIPng process called Mariners into OSPFv3 process ID 1 with a metric-type of external, type 1. Also, a tag value of 4 is added to these routes. The tag can later be used to identify routes advertised by this RIPng process. Boone's IPv6 route table (Example 11-35) shows these routes.

Example 11-35. IPv6 route table shows routes redistributed into OSPF from RIP, with a tag of 4 and external type 1 metrics.

Boone#show ipv6 route ospf IPv6 Routing Table - 26 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 OE1  2001:DB8:0:22::/64 [110/84], tag 4      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE1  2001:DB8:0:23::/64 [110/84], tag 4      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE1  2001:DB8:0:24::/64 [110/84], tag 4      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE1  2001:DB8:0:25::/64 [110/84], tag 4      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE1  2001:DB8:0:26::/64 [110/84], tag 4      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE1  2001:DB8:0:27::/64 [110/84], tag 4      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 Boone#

Griffey advertises the RIPng routes into OSPFv3 process ID 2 with the default metric-type (external type 2) and no tag value. Garcia's route table (Example 11-36) shows the redistributed routes.

Example 11-36. IPv6 route table shows redistributed routes with the default metric-type of external type 2.

Garcia#show ipv6 route ospf IPv6 Routing Table - 26 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 OE2  2001:DB8:0:22::/64 [110/20]      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE2  2001:DB8:0:23::/64 [110/20]      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE2  2001:DB8:0:24::/64 [110/20]      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE2  2001:DB8:0:25::/64 [110/20]      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE2  2001:DB8:0:26::/64 [110/20]      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 OE2  2001:DB8:0:27::/64 [110/20]      via  FE80::207:85FF:FE6B:EA20, Serial0/0.1 Garcia#

The subnets keyword is not used with OSPFv3 for IPv6. All IPv6 routing protocols include IPv6 prefixes, which always include the address and the prefix length. Redistribution between protocols includes all prefixes unless filtering is performed to limit the prefixes advertised. Route filtering is discussed in Chapter 13 and Chapter 14.

Griffey also redistributes the routes known via both OSPFv3 processes into RIPng. OSPF 1 routes are advertised with a metric of 5 and OSPF 2 routes are advertised with a metric of 10. Example 11-37 shows Suzuki's IPv6 route table.

Example 11-37. The RIPng route table shows routes redistributed into RIPng with different metrics.

Suzuki#show ipv6 route rip IPv6 Routing Table - 19 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 R   2001:DB8:0:11::/64 [120/11]      via FE80::207:85FF:FE6B:EA20, Ethernet0/0 R   2001:DB8:0:12::/64 [120/11]      via FE80::207:85FF:FE6B:EA20, Ethernet0/0 R   2001:DB8:0:13::/64 [120/11]      via FE80::207:85FF:FE6B:EA20, Ethernet0/0 R   2001:DB8:0:14::/64 [120/11]      via FE80::207:85FF:FE6B:EA20, Ethernet0/0 R   2001:DB8:0:AA01::/64 [120/6]      via FE80::207:85FF:FE6B:EA20, Ethernet0/0 R   2001:DB8:0:AA02::/64 [120/6]      via FE80::207:85FF:FE6B:EA20, Ethernet0/0 R   2001:DB8:0:AA03::/64 [120/6]      via FE80::207:85FF:FE6B:EA20, Ethernet0/0 Suzuki#

The routes from Boone have a metric of 6 and the routes from Garcia have a metric of 11.

Route summarization is now going to be configured on Griffey. RIPng does not have a method for summarizing the routes that are advertised into it when redistributing from another protocol, but Griffey's Ethernet interface connecting to Suzuki can be configured to summarize the RIPng prefixes that are advertised out the interface. OSPFv3 summarizes the RIPng prefixes as they are redistributed into the routing process. Griffey's configuration is displayed in Example 11-38.

Example 11-38. Griffey summarizes RIPng routes as they are advertised out Ethernet0/0, and also summarizes routes as they are redistributed into OSPFv3.

interface Ethernet 0/0  ipv6 address 2001:db8:0:21::1/64  ipv6 rip Mariners enable  ipv6 rip Mariners summary-address 2001:DB8:0:AA00::/62  ipv6 rip Mariners summary-address 2001:DB8:0:10::/61 ! interface Serial 0/0.1 point-to-point  ipv6 address 2001:db8:0:31::1/64  ipv6 ospf 1 area 1 ! interface Serial 0/0.4 point-to-point  ipv6 address 2001:db8:0:30::1/64  ipv6 ospf 2 area 20 ! ipv6 router ospf 1  redistribute rip Mariners metric-type 1 tag 4  summary-prefix 2001:DB8:0:20::/61 tag 4 ! ipv6 router ospf 2  redistribute rip Mariners  summary-prefix 2001:DB8:0:20::/61 ! ipv6 router rip Mariners  redistribute ospf 1 metric 5  redistribute ospf 2 metric 10

The new route tables show the summarized routes and show that the more specific routes have been removed (Example 11-39).

Example 11-39. Route tables include summarized routes but no longer include the more specific routes that fall into the summarized range.

Boone#show ipv6 route ospf IPv6 Routing Table - 21 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 OE1  2001:DB8:0:20::/61 [110/84], tag 4      via FE80::207:85FF:FE6B:EA20, Serial0/0.1 Boone# ___________________________________________________________________________ Garcia#show ipv6 route ospf IPv6 Routing Table - 21 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 OE2  2001:DB8:0:20::/61 [110/20]      via FE80::207:85FF:FE6B:EA20, Serial0/0.1 Garcia# ___________________________________________________________________________ Suzuki#show ipv6 route rip IPv6 Routing Table - 19 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 R    2001:DB8:0:10::/61 [120/11]       via FE80::207:85FF:FE6B:EA20, Ethernet0/0 R    2001:DB8:0:20::/61 [120/6]       via FE80::207:85FF:FE6B:EA20, Ethernet0/0 R    2001:DB8:0:AA00::/62 [120/6]       via FE80::207:85FF:FE6B:EA20, Ethernet0/0 Suzuki#

Notice that the summary route 2001:db8:0:20::/61 is being advertised by Griffey to Suzuki. This is the summary route created for the prefixes advertised into OSPFv3. This summary route is being advertised from OSPF back into RIPng. The routers in the RIPng domain will still be able to route to the more specific prefixes correctly. Suzuki's full IPv6 route table (Example 11-40) shows that the prefixes that are included in the summary 2001:db8:0:20::/61 are directly connected to the router. The router will forward traffic to the more specific prefixes, rather than to the summary. A downside of having this summary re-advertised back into the RIPng domain is that packets destined to an address that falls into the summary range, but does not actually exist in the network, will get forwarded to Griffey before being dropped. The route can be filtered during route redistribution so that only prefixes known to reside in the OSPFv3 domain are advertised into the RIPng domain. Route filtering during redistribution is discussed in Chapter 13 and Chapter 14.

Example 11-40. Suzuki's RIPng route table shows a summary route and more specific routes that fall within the summary route's range. The router will forward traffic to the more specific routes when possible.

Suzuki#show ipv6 route IPv6 Routing Table - 19 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 R    2001:DB8:0:10::/61 [120/11]       via FE80::207:85FF:FE6B:EA20, Ethernet0/0 R    2001:DB8:0:20::/61 [120/6]       via FE80::207:85FF:FE6B:EA20, Ethernet0/0 C    2001:DB8:0:21::/64 [0/0]       via ::, Ethernet0/0 L    2001:DB8:0:21::2/128 [0/0]       via ::, Ethernet0/0 C    2001:DB8:0:22::/64 [0/0]       via ::, Serial0/0 L    2001:DB8:0:22::1/128 [0/0]       via ::, Serial0/0 C    2001:DB8:0:23::/64 [0/0]       via ::, Serial0/0 L    2001:DB8:0:23::1/128 [0/0]       via ::, Serial0/0 C    2001:DB8:0:24::/64 [0/0]       via ::, Serial0/0 L    2001:DB8:0:24::1/128 [0/0]       via ::, Serial0/0 C    2001:DB8:0:25::/64 [0/0]       via ::, Serial0/0 L    2001:DB8:0:25::1/128 [0/0]       via ::, Serial0/0 C    2001:DB8:0:26::/64 [0/0]       via ::, Serial0/0 L    2001:DB8:0:26::1/128 [0/0]       via ::, Serial0/0 C    2001:DB8:0:27::/64 [0/0]       via ::, Serial0/0 L    2001:DB8:0:27::1/128 [0/0]       via ::, Serial0/0 R    2001:DB8:0:AA00::/62 [120/6]       via FE80::207:85FF:FE6B:EA20, Ethernet0/0 L    FE80::/10 [0/0]       via ::, Null0 L    FF00::/8 [0/0]       via ::, Null0 Suzuki#

Case Study: Redistributing IS-IS and RIP/RIPng

In the network of Figure 11-12, Aaron is running IS-IS, Williams is running RIPv1 for IPv4 and RIPng for IPv6, and Mays is redistributing.

Figure 11-12. Router Mays is redistributing RIP into IS-IS and IS-IS into RIP.

Mays's IS-IS and RIP IPv4 and IPv6 configuration is displayed in Example 11-41.

Example 11-41. Mays is configured for IS-IS, RIP, and RIPng.

interface Ethernet1  description to Williams  ip address 10.2.1.1 255.255.255.0  ipv6 address 2001:db8:0:21::1/64  ipv6 rip baseball enable ! interface Ethernet0  description to Aaron  ip address 10.1.4.2 255.255.255.0  ip router isis  ipv6 address 2001:db8:0:14::2/64  ipv6 router isis router rip  passive-interface Ethernet0  network 10.0.0.0  redistribute isis level-1-2 metric 1 ! router isis  net 01.0001.0000.0c76.5432.00  redistribute rip metric 0 metric-type internal level-2  address-family ipv6   redistribute rip baseball metric 0 metric-type internal level-2 ! ipv6 router rip baseball  redistribute isis level-1-2 metric 1

Routes may be redistributed into IS-IS with either internal or external metrics (internal is the default) and as either level 1 or level 2 routes (level 2 is the default). The metric type determines the base metric value of the redistributed routes. The metric value of an internal metric type will be lower then 64. The metric value of an external metric will be between 64 and 128. If the metric type is internal, the initial metric of the redistributed route will be the number specified by the metric value (0 in this example), or 0 if no metric is specified. The redistributed route in this example gets added to Mays's IS-IS database entry with a metric of 0. If the metric value of 5 were specified, the entry in Mays's IS-IS database would be 5. An external type of metric starts with a base value of 64. If the metric value of 5 is specified, the metric value for the entry in Mays's IS-IS database would be 64 + 5, or 69. In the example shown, the RIP routes are redistributed as internal, level 2 routes with the default metric of 0. Example 11-42 shows the redistributed routes in Aaron's IPv4 route table, and Example 11-43 shows the redistributed route in the IPv6 route table.

Example 11-42. Aaron's IPv4 route table shows the redistributed RIP routes.

Aaron#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks C     10.1.3.0/24 is directly connected, Ethernet4 i L2  10.2.1.0/24 [115/10] via 10.1.4.2, Ethernet0 i L2  10.2.2.0/24 [115/10] via 10.1.4.2, Ethernet0 C     10.1.1.0/24 is directly connected, Ethernet1 C     10.1.4.0/24 is directly connected, Ethernet0 C     10.1.2.160/28 is directly connected, Ethernet2 C     10.1.2.224/28 is directly connected, Ethernet3 Aaron#

Example 11-43. Aaron's IPv6 route table shows the redistributed RIP routes.

Aaron#show ipv6 route isis IPv6 Routing Table - 13 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B  BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 I2 2001:DB8:0:22::/64 [115/10]     via FE80::204:C1FF:FE50:E700, Ethernet0 Aaron#

The IPv6 route table on Aaron shows that the address connected to Mays, 2001:db8:0:21::/64, is not advertised into IS-IS. An additional command, redistribute connected, is added to Mays in Example 11-44 to redistribute the connected addresses, and the resulting IPv6 route table is shown in Example 11-45.

Example 11-44. redistribute connected is added to Mays, so the directly connected IPv6 prefix can be advertised into IS-IS.

router isis  net 01.0001.0000.0c76.5432.00  redistribute rip metric 0 metric-type internal level-2  address-family ipv6   redistribute rip baseball metric 0 metric-type internal level-2   redistribute connected metric 0 metric-type internal level-2 ! ipv6 router rip baseball  redistribute isis level-1-2 metric 1  redistribute connected metric 1

Example 11-45. Aaron's IPv6 route table shows redistributed RIP and connected addresses.

Aaron#show ipv6 route isis IPv6 Routing Table - 14 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B  BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 I2  2001:DB8:0:21::/64 [115/10]      via FE80::204:C1FF:FE50:E700, Ethernet0 I2  2001:DB8:0:22::/64 [115/10]      via FE80::204:C1FF:FE50:E700, Ethernet0 Aaron#

Because the RIP routes are external to the IS-IS routing domain, it is best to reflect this by having them redistributed into the domain as external routes, as in Example 11-46.

Example 11-46. Mays configures the routes redistributed from RIP to IS-IS to external routes.

router isis  redistribute rip metric 0 metric-type external level-2  net 01.0001.0000.0c76.5432.00  address-family ipv6   redistribute rip baseball metric 0 metric-type external level-2   redistribute connected metric 0 metric-type external level-2 ! router rip  redistribute isis level-1-2 metric 1  passive-interface Ethernet0  network 10.0.0.0 ! ipv6 router rip baseball  redistribute isis level-1-2 metric 1  redistribute connected metric 1

Example 11-47 shows Aaron's IPv4 route table after the change. The only change from Example 11-42 is that the metrics of the redistributed routes have increased to greater than 64, indicating (in this small network) external routes. Example 11-48 displays the IS-IS database entry for Mays. The entries for the redistributed routes are shown as IP-External and IPv6-Ext.

Example 11-47. The metrics of the routes to 10.2.1.0/24 and 10.2.2.0/24 change to 138 after the routes are advertised as external.

Aaron#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B  BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2        ia - IS-IS inter area, * - candidate default, U - per-user static route        o - ODR, P - periodic downloaded static route Gateway of last resort is not set        10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks C         10.1.3.0/24 is directly connected, Ethernet4 i L2      10.2.1.0/24 [115/74] via 10.1.4.2, Ethernet0 i L2      10.2.2.0/24 [115/74] via 10.1.4.2, Ethernet0 C         10.1.1.0/24 is directly connected, Ethernet1 C         10.1.4.0/24 is directly connected, Ethernet0 C         10.1.2.160/28 is directly connected, Ethernet2 C         10.1.2.224/28 is directly connected, Ethernet3 Aaron#

Example 11-48. Redistributed routes are displayed as IP-External and IPv6-Ext in Mays's IS-IS database.

Mays# show isis database detail level-2 Mays.00-00 IS-IS Level-2 LSP Mays.00-00 LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime     ATT/P/OL Mays.00-00          * 0x0000005F   0xA0E5        1186             0/0/0   Area Address: 01.0001   NLPID:        0xCC 0x8E   Hostname:     Mays   IP Address:   10.1.4.2   IPv6 Address: 2001:DB8:0:14::2   Metric:10          IS Aaron.00   Metric:20          IP 10.1.3.0 255.255.255.0   Metric:64          IP-External 10.2.1.0 255.255.255.0   Metric:64          IP-External 10.2.2.0 255.255.255.0   Metric:20          IP 10.1.1.0 255.255.255.0   Metric:10          IP 10.1.4.0 255.255.255.0   Metric:20          IP 10.1.2.160 255.255.255.240   Metric:20          IP 10.1.2.224 255.255.255.240   Metric:20          IPv6 2001:DB8:0:11::/64   Metric:20          IPv6 2001:DB8:0:13::/64   Metric:10          IPv6 2001:DB8:0:14::/64   Metric:0           IPv6-Ext 2001:DB8:0:21::/64   Metric:0           IPv6-Ext 2001:DB8:0:22::/64   Metric:20          IPv6 2001:DB8:0:12A::/64   Metric:20          IPv6 2001:DB8:0:12B::/64 Mays#

Another look at Figure 11-12 shows that both subnets of the RIP domain, IPv4 and IPv6, can be summarized with a single address of 10.2.0.0/16 and 2001:db8:0:20::/62. IPv4 route summarization into IS-IS is configured with the same summary-address command that is used with OSPF. However, the level into which the summary is being sent can also be specified. If the level is not specified, the addresses will be summarized into level 2. As with OSPFv3 for IPv6, IPv6 routes are summarized into IS-IS using the summary-prefix command for the IPv6 address-family. In the configuration in Example 11-49, RIP IPv4 routes are redistributed as level 1 and summarized, and RIPng IPv6 routes are redistributed as level 2 and summarized.

Example 11-49. Mays summarizes the RIP routes after they have been redistributed into IS-IS.

router isis  summary-address 10.2.0.0 255.255.0.0 level-1  redistribute rip metric 0 metric-type external level-1  net 01.0001.0000.0c76.5432.00  address-family ipv6   redistribute rip baseball metric 0 metric-type external level-2   redistribute connected metric 0 metric-type external level-2   summary-prefix 2001:db8:0:20::/62 level-2 router rip  redistribute isis level-1-2 metric 1  passive-interface Ethernet0  network 10.0.0.0 ! ipv6 router rip baseball  redistribute isis level-1-2 metric 1  redistribute connected metric 1

Example 11-50 shows the summary route in Aaron's IPv4 route table. Example 11-51 shows Aaron's IPv6 route table. Like OSPF and EIGRP, the summarization causes more-specific routes in the summary range to be suppressed.

Example 11-50. Aaron's IPv4 route table with a level-1 summary route to the subnets within the RIP domain.

Aaron#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      10.0.0.0/8 is variably subnetted, 6 subnets, 3 masks i L1    10.2.0.0/16 [115/138] via 10.1.4.2, Ethernet0 C       10.1.3.0/24 is directly connected, Ethernet4 C       10.1.1.0/24 is directly connected, Ethernet1 C       10.1.4.0/24 is directly connected, Ethernet0 C       10.1.2.160/28 is directly connected, Ethernet2 C       10.1.2.224/28 is directly connected, Ethernet3 Aaron#

Example 11-51. Aaron's IPv6 route table with a level-2 summary route to the subnets within the RIP domain.

Aaron#show ipv6 route isis IPv6 Routing Table - 13 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B  BGP        U - Per-user Static route        I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary        O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2        ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 I2  2001:DB8:0:20::/62 [115/10]      via FE80::204:C1FF:FE50:E700, Ethernet0 Aaron#

When IS-IS is redistributed into another protocol, the route level to be redistributed must be specified. In the examples shown so far, both level 1 and level 2 routes have been specified to be redistributed into RIP.

Case Study: Redistributing Static Routes

Example 11-52 shows the route table of Williams in Figure 11-12. Notice that subnets 10.1.2.160/28 and 10.1.2.224/28 are missing; their subnet masks are inconsistent with the 24-bit mask configured on Mays's E1 interface, so the routes have not been included in the RIP updates sent out that interface. This scenario again illustrates the problem of redistributing variably subnetted routes from a classless protocol into a classful protocol, as discussed earlier in this chapter.

Example 11-52. The routes with subnets other than /24 are not redistributed into the RIP domain.

Williams#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route Gateway of last resort is not set      10.0.0.0/8 is subnetted, 5 subnets R       10.1.3.0 [120/1] via 10.2.1.2, 00:00:01, Ethernet0 C       10.2.1.0 is directly connected, Ethernet0 R       10.1.1.0 [120/1] via 10.2.1.2, 00:00:02, Ethernet0 C       10.2.2.0 is directly connected, Ethernet1 R       10.1.4.0 [120/1] via 10.2.1.2, 00:00:02, Ethernet0 Williams#

A solution to this problem is to summarize the two 28-bit subnets with a single 24-bit address of 10.1.2.0/24. RIP does not have a summarization command, so the way to accomplish summarization is to configure a static route to the summary address and then redistribute that route into RIP, as displayed in Example 11-53.

Example 11-53. Mays is configured with a static route to create a summarized route for IPv4 subnets. The static route is redistributed into RIP.

router isis  summary-address 10.2.0.0 255.255.0.0 level-1  redistribute rip metric 0 metric-type external level-1  net 01.0001.0000.0c76.5432.00 ! router rip  redistribute static metric 1  redistribute isis level-1-2 metric 1  passive-interface Ethernet0  network 10.0.0.0 ! ip route 10.1.2.0 255.255.255.0 10.1.4.1

Example 11-54 shows Williams's route table with the summary route included.

Example 11-54. Subnets 10.1.2.160/28 and 10.1.2.224/28 have been summarized with the address 10.1.2.0/24.

Williams#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route Gateway of last resort is not set    10.0.0.0/8 is subnetted, 6 subnets R    10.1.3.0 [120/1] via 10.2.1.2, 00:00:03, Ethernet0 R    10.1.2.0 [120/1] via 10.2.1.2, 00:00:03, Ethernet0 C    10.2.1.0 is directly connected, Ethernet0 R    10.1.1.0 [120/1] via 10.2.1.2, 00:00:03, Ethernet0 C    10.2.2.0 is directly connected, Ethernet1 R    10.1.4.0 [120/1] via 10.2.1.2, 00:00:03, Ethernet0 Williams#

As discussed in Chapter 3, a variant of the static route is an entry that points to an outgoing interface instead of a next-hop address. These static routes can also be redistributed, but the configuration is somewhat different. For example, the configuration of Mays can be as displayed in Example 11-55.

Example 11-55. Mays is configured with a static route pointing out an interface rather then to a next-hop address.

router isis  summary-address 10.2.0.0 255.255.0.0 level-1  redistribute rip metric 0 metric-type external level-1  net 01.0001.0000.0c76.5432.00 ! router rip  redistribute isis level-1-2 metric 1  passive-interface Ethernet0  network 10.0.0.0 ! ip route 10.1.2.0 255.255.255.0 Ethernet0

Here the static route now points to Mays's E0 interface instead of to the next-hop address 10.1.4.1. Also, the redistribute static command is no longer used under the RIP configuration, yet Williams's route table still looks the same as in Example 11-54.

The reason this static route still gets redistributed is that when a static route points to an outgoing interface, the destination is considered by the router to be directly connected (Example 11-56). And because a network statement for 10.0.0.0 appears under the RIP configuration, RIP will advertise this "directly connected" subnet of 10.0.0.0.

Example 11-56. Mays considers the summary address 10.1.2.0/24 to be directly connected to Ethernet 0.

Mays#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks i L1    10.1.3.0/24 [115/20] via 10.1.4.1, Ethernet0 S       10.1.2.0/24 is directly connected, Ethernet0 C       10.2.1.0/24 is directly connected, Ethernet1 i L1    10.1.1.0/24 [115/20] via 10.1.4.1, Ethernet0 R       10.2.2.0/24 [120/1] via 10.2.1.1, 00:00:21, Ethernet1 C       10.1.4.0/24 is directly connected, Ethernet0 i       L1 10.1.2.160/28 [115/20] via 10.1.4.1, Ethernet0 i       L1 10.1.2.224/28 [115/20] via 10.1.4.1, Ethernet0 Mays#

Suppose Williams receives a packet with a destination address of 10.1.2.5. The summary address of 10.1.2.0/24 will be matched, and the packet will be forwarded to Mays. At Mays, the destination address does not match a more-specific subnet, and therefore will match the static route. Mays will send ARP requests out E0 in an attempt to find host 10.1.2.5 (or a router that will send a Proxy ARP reply). Finding none, the router does not know what to do with the packet. An ICMP Destination Unreachable message will not be sent to the originator.

Recall that when the summarization commands are used, they create an entry in the route table to Null 0.

Note

Null interfaces should be configured in conjunction with static summary routes.

The same can and should be done with static summary routes (see Example 11-57).

Example 11-57. Mays's static summary route now points to the null interface.

router isis  summary-address 10.2.0.0 255.255.0.0 level-1  redistribute rip metric 0 metric-type external level-1  net 01.0001.0000.0c76.5432.00 ! router rip  redistribute isis level-1-2 metric 1  passive-interface Ethernet0  network 10.0.0.0 ! ip route 10.1.2.0 255.255.255.0 Null0

Now any destination address that doesn't find a more-specific match at Mays will be routed to the null interface and dropped, and an ICMP Destination Unreachable message will be sent to the originator.